There are many instances when it is desirable to dissolve a gas, whether soluble or insoluble, into a fluid that may already contain other dissolved gases. For example, the macro and microbial organisms in all rivers, lakes, oceans, and all aerobic wastewater treatment processes are based on the presence of sufficient dissolved oxygen to sustain their life processes. Normally, in undisturbed bodies of water there is a rather low density of macro and micro organisms in the surface water and the limited natural absorption of oxygen from the air into the water is sufficient to maintain sufficient concentrations of dissolved oxygen in the water to sustain the life processes of that body of water. However, with increased population density and industrial activity, the associated organic water pollution causes a high microbial oxygen demand that natural oxygen aeration processes cannot begin to provide sufficient oxygen resources. Thus, artificial aeration mechanisms are required to enhance oxygen absorption.
Some specific examples of oxygenation applications are worthy of discussion. Odors at aerobic wastewater treatment facilities are associated with the inability to maintain sufficient levels of dissolved oxygen (“D.O.”). In the absence of sufficient D.O., nitrates are reduced to N2 gas. In the absence of both D.O. and nitrates, strongly reducing conditions develop and sulfates are reduced to H2S, also known as “rotten egg gas”. This process can occur in any aquatic system where the oxygen demand exceeds the D.O. supply.
The high organic pollution in municipal wastewater of sewer lift stations supports a corresponding high microbial population, which, in turn, requires a high rate of D.O. to meet the demand. If the demand is not met, H2S formation readily occurs. Consequently, sewer force mains are a common source of odor nuisance for municipal public works.
Some industries (pharmaceutical, petroleum, and chemical, for example) create significant air pollution problems in the course of aerobically treating their wastewater by the use of conventional aeration systems. The wastewaters contain significant volatile organics/solvents that are readily biodegradable if they can be retained in the aqueous phase for a sufficient time. The use of conventional aeration systems has led to the requirement that the wastewater aeration basins must be covered to capture and incinerate the off gas in order to comply with air emission regulations. The need for a covered basin arises because conventional aeration systems readily strip the organics/solvents from the aqueous phase, not allowing for a sufficient time to biograde in the liquid.
Aerobic activated sludge processes are dependent upon oxygen transfer and sludge settling and recycle in the secondary clarifiers. It is now possible to develop high concentrations of sludge concentrations within the reactors, such as with the use of aerobic fluidized beds and moving bed filters, to the point where oxygen transfer becomes the limiting factor. Specifically, high levels of D.O. are required without subjecting the sludge to high energy dissipation/turbulence conditions that could shear off the biofilms or hinder flocculent sedimentation in the secondary clarifiers.
Fish farming and shrimp production commonly occurs in large ponds. To maximize production, the ponds are operated at the edge of D.O. availability. Since a still pond absorbs very little oxygen, there exists a need for artificial aeration to sustain high levels of fish/shellfish production.
The desire to increase dissolved oxygen levels is also applicable to slow moving rivers (such as the Cuyahoga River flowing through Cleveland, Ohio, and the rivers in Bangkok and Taipei) and canals (such as the waterways of Chicago, Ill. and the canals of Amsterdam). Many industries must curtail production (to considerable economic detriment) due to insufficient D.O. in the rivers, streams, and canals to which they discharge their treated wastewaters. Odor and corrosion problems can also occur in the bottom layer of stratified lakes and reservoirs feeding hydroelectric power dams. The low D.O. levels also result in fish kills.
Systems for dissolving a gas into a fluid are not limited to dissolving oxygen in water. Other gas/fluid combinations include: hydrogenation of vegetable oils, coal liquification, yeast production, Vitamin C production, pharmaceutical and industrial aerobic bioprocesses, and other processes well known in the art.
When high purity oxygen is being transferred into water, issues arise as to handling of dissolved nitrogen (“D.N.”) already in the water. D.N. is not utilized in an aqueous environment. Air is primarily comprised of 21% oxygen and 79% nitrogen gas. When water is in contact with air for prolonged periods, the water is saturated with D.N. At 20° C., the saturation concentration of D.N. in water is 16 mg/L. With conventional aeration systems, D.N. levels remain in a steady state. However, when high purity oxygen is introduced into the water, it results in a reduced D.N. partial pressure that strips the D.N. from the dissolved phase into the gas phase where it, in turn, reduces the percentage oxygen composition. The reduction in percentage oxygen composition reduces the partial pressure of oxygen in the gas phase, and the saturation concentration of oxygen, and ultimately the rate of oxygen transfer.
Thus, the presence of D.N. in the incoming water presents a trade-off situation. If high oxygen absorption efficiency is to be achieved, the increased nitrogen gas composition in the gas phase has to be accepted. This reduces the D.O. concentration which can be achieved in the discharge. Conversely, if high D.O. levels are to be achieved in the discharge, then the stripped nitrogen in the gas phase has to be wasted to reduce its percentage composition carrying with it a commensurate ratio of oxygen gas and reducing the percentage oxygen absorption efficiency.
It is well known that pressure greatly enhances the dissolving of a gas into a liquid, including, but not limited to, dissolving high purity oxygen (HPO) into water. However, in conventional pressurization schemes, considerable energy expenditure is involved. To dissolve HPO into water, the rate of gas transfer is related to partial pressure of the gas to be transferred, Csat, which is dependent upon the pressure of the gas to be transferred. The partial pressure of the gas to be transferred may be computed by multiplying the total pressure times the composition of the gas in the gas phase:Csat=(45 mg/L-atmosphere)(composition of gas phase)(pressure of the gas phase) For the case of dissolving oxygen into water:Csat=(45 mg/L-atmosphere)(Oxygen partial pressure in atmosphere) For air at one atmosphere of pressure, the oxygen fraction is 0.21 and thus the total oxygen partial pressure is 0.21 atmosphere. Thus, at one atmosphere of pressure,Csat=(45×0.21=9.2 mg/L. For 100% oxygen at 15 psig, the partial pressure is 2.0 atmospheres absolute and Csat is 90 mg/L. Thus, increases in purity and pressure of the gas to be dissolved significantly increases Csat.
The rate of gas transfer, dc/dt, is related to a number of factors as shown in the gas transfer equation:dc/dt=K1(A/V)(Csat−Csat) where                K1 is the gas transfer coefficient        A is the interfacial area of gas exposed to the water        V is the volume of water        Csat is the saturation concentration as defined above        Cact is the actual concentration of dissolved gas in the water        
Since the liquid into which the high purity gas is to be transferred often contains other dissolved gases, these extraneous gases are stripped from solution into the gas phase because the liquid is supersaturated with the extraneous gas relative to the gas phase and therefore transfer is out of the liquid into the gas.
Water as it exists in the environment is in contact with air that not only contains oxygen gas but also nitrogen, carbon dioxide, argon and other trace gases. Many micro and macro organisms utilize oxygen in their life processes, so the water may or may not be saturated with oxygen. However, because nitrogen gas and other gases are conservative gases, these other gases are not utilized. Conservative gases are normally always at saturation conditions in the waters of the environment. Consequently, when HPO is dissolved into water, any other gas already dissolved in the water will be stripped from the liquid phase into the gas phase. This results in a dilution of the HPO in the gas phase and lowers the oxygen composition of the gas phase, causing a reduction of Csat, and a corresponding reduction in the rate of gas transfer.
Nitrogen gas in air has a partial pressure of 0.79 atmospheres, and, therefore, at equilibrium has a dissolved partial pressure of 0.79 atmospheres. At sea level and 20° C., this saturation concentration of nitrogen gas dissolved in water in equilibrium with air is about 16 mg/L of dissolved nitrogen. Thus, if a bubble of pure oxygen is held in contact with water at saturation level with air, the oxygen composition will reduce from 100% oxygen to ultimately only 21% oxygen (if 100% oxygen absorption efficiency is achieved) by the combined process of oxygen being absorbed from the bubble into the water and nitrogen gas being stripped from solution into the bubble.
If the oxygen absorption efficiency is less than 100%, some of the gas will be wasted from the system, carrying with it some of the oxygen as well as nitrogen stripped from solution. In such a case, the equilibrium oxygen composition of the bubble would be above 21%.
There are advantages to be gained by conducting the gas transfer of the high purity gas into the liquid under elevated pressure. Two major advantages accrue to such a procedure: 1) pressure increases in the gas phase to be dissolved resulting in a concomitant increase in Csat; and 2) the stripping potential of the extraneous gases, such as nitrogen, already dissolved in the liquid are reduced.
For instance, if 100% O2 is to be dissolved into water which is at the air saturation with nitrogen at ambient pressure this reduces Csat from 45 mg/L to 9.2 mg/L with a comparable reduction in the rate of oxygen transfer. However, if the pure oxygen bubble is held in contact with water saturated with air at a pressure of 34 ft (2.0 atmospheres absolute) below the surface, the equilibrium composition of nitrogen gas in the bubble now only approaches 39.5% if 100% oxygen absorption was achieved (79%/2 atmospheres). This, of course, is the same partial pressure of nitrogen gas. However, the oxygen composition would now reach an equilibrium value of approximately 60.5% instead of the 21% at ambient pressure. Consequently the corresponding Csat for a pressure of 34 ft would be:Csat=atmospheres (0.605 O2 composition) 45 mg/L per atmosphere of partial pressure=54 mg/L D.O. If the gas transfer reactor was at 45 psig (4.0 atmospheres absolute) the nitrogen in the gas phase would approach only approach 20% (79%/4). The oxygen composition would then be 80% and the resulting Csat would be:Csat=4 atmospheres (0.80 O2 composition)45 mg/L per atmosphere=144 mg/L Pressure simultaneously reduces nitrogen gas stripping and increases the Csat of O2. Not only is the rate of gas transfer greatly increased, but also the concentration of D.O. that can be reached in the discharge is also higher.
Even though pressurization can greatly enhance the gas transfer process, it unfortunately also increases the energy consumption required to dissolve the oxygen (in terms of kWatt-hr/ton of D.O.). For instance, it is relatively easy to pressurize a gas transfer chamber by pumping against a backpressure maintained in the chamber with a throttling valve at the discharge. This is the method incorporated in conventional dissolved air floatation processes. Referring now to FIG. 1, there is shown a schematic diagram of an aeration system according to the prior art. Prior art aeration system 10 includes reactor 12, liquid (water) feed inlet 14, feed pump 16 for moving liquid (water) into reactor 12 through feed inlet 14, gas (oxygen) feed inlet 18 for introducing gas (oxygen) into reactor 12, outlet 20, and throttling valve 22. Throttling value 22 is controlled to maintain the desired pressure within reactor 12. While the embodiment of FIG. 1 greatly enhances gas transfer by increasing the pressure within reactor 12, prior art system 10 also greatly increases unit energy consumption. Considerable energy is consumed by pressurization pump 16 to force liquid (water) into reactor 12. The energy transferred into system 10 by pump 16 is later lost at throttling valve 22 when the pressure in reactor 12 is reduced to ambient pressure. It is therefore desired to employ a method of pressurization that minimizes net energy consumption.
The U-Tube is a prior art apparatus used to hydrostatically pressurize the gas transfer process with minimal input of energy. The water is directed vertically down a shaft underneath a baffle and flows back to the surface. Little energy is required to move the water through the U-Tube, but the hydrostatic pressurization can be considerable. For instance, if a U-Tube is 175 ft deep, the hydrostatic pressure at the bottom reaches 6 atmospheres absolute pressure (175/34+1=6). If HPO is injected at the inlet of the U-Tube and is swept down and back up out, the Csat can reach 230 mg/L at the bottom (87% O2 times 6 atmospheres times 45 mg/L-atmospheres partial pressure). Thus, considerable increase in the rate of oxygen transfer can be achieved in a U-Tube oxygen transfer system with little energy consumption expended. However, one disadvantage with use of the U-Tube in that is requires the drilling of a deep shaft that presents a problem particularly if rock is encountered at the site. In such cases, the drilling of the shaft can cost several hundred thousand dollars, and, in many instances, is prohibitively expensive.
It is therefore desired to provide a gas transfer system, namely, a system for dissolving at least one gas into a fluid, that is energy efficient. It is also desired to provide such a system that is comprised of reliable components, and that is reasonable in maintenance and repair costs. It is further desired to provide a system that does not require drilling of a deep shaft as is required with the aforementioned U-Tube configuration.
Laboratory experiments have demonstrated that 80 to 120 mg/L of D.O. can be kept in solution without effervescent loss. Effervescent loss of dissolved gas is related to the level of supersaturation of the dissolved gas, the turbulence in the water, and time for loss to take place. There is a threshold level of supersaturation below which effervescent loss cannot occur. Furthermore, above this threshold level, there is a corresponding level of turbulence and time required to make the effervescence occur.
For instance, in dissolved air flotation, the water is aerated in a pressure vessel at 45 to 75 psig (because effervescence is not significant if the pressure is below about 30 psig). Subsequently, it is released through the throttling valve employed to create the backpressure. At 45 psig (4 atmospheres absolute) the water may be near 400% total saturation. However, it takes the violent turbulence in the throttling to make it effervesce. Without this violent turbulence, it effervesces inefficiently.
If water is oxygenated under pressure with high purity oxygen to produce 40 mg/L D.O. and there is negligible change in the dissolved nitrogen concentration, the total dissolved gas pressure will be 0.79 atmospheres of N2 and (40/9.2×0.21 atmospheres) 0.91 atmospheres of O2, for a total of 1.70 atmospheres of dissolved gas. For a D.O. of 80 mg/L, and again ignoring the minor dissolved nitrogen gas stripping under pressurized conditions, the total dissolved gas pressure would be 0.79+(80/9.2×0.21)=2.6 atmospheres of total dissolved gas. As previously mentioned, in laboratory experiments, effervescence was not observed in oxygenated water containing 80 mg/L D.O. This finding implies that the level of supersaturation must exceed 260% to support effervescence.
It is therefore desired to provide a gas transfer system with energy recovery capabilities that also reduces effervescence loss.